1. Field of the Invention
This invention relates to a data communication system so as to form a loop data highway system.
2. Technical Considerations and Prior Art
In a loop data highway system in which a plurality of data stations are connected in series by a common transmission line to form a so called highway. The transmission request signals for the highway, concurrently generated by a plurality of data stations must be multiplexed or processed on a time sharing basis. Generally, the time sharing system in which definite time slots are allocated to a data station by prefixing addresses to the information has a higher transmission efficiency and flexibility than a time division multiplex system in which identical time slots are fixedly allocated to the data stations. For this reason, the former system has also been applied to data communication systems having relatively small scale data processing fields. The system performance can be improved greatly by adopting a priority control over the transmission requests generated for the same time slot by a plurality of data stations. A station generating a transmission request having the highest priority level is permitted to use the time slots and a priority level may be given in accordance with the state of transmission request.
Where information is transmitted according to a fixed time slot defined by a central data station, the central data station receives the information in time slots accompanied with a loop delay of propagation, that include the information that must be retransmitted at the next time slots. As the loop delay variation is mainly caused by the length of the highway and the length is differs in accordance with different systems the loop delay consequently varies for different systems. Accordingly, it is necessary to add an additional delay in order to get the proper time for retransmission at the central data station.